Inter- and intra-limb bone length and strength proportions are indicative of adult locomotor behavior. However, locomotor behavior often changes through ontogeny in association with changes in body size and proportions. As prior work has shown, infant mountain gorillas (Gorilla beringei beringei) experience a phase of increased arboreality before shifting into adult-like, terrestrial knuckle walking; as adults, they are less arboreal than lowland gorillas (Gorilla gorilla gorilla).

In this study we examined inter- and intra-limb bone length and strength proportions of an ontogenetic series of mountain gorillas (n = 34) and adult lowland gorillas (n = 13). Bone strength (polar section modulus) was determined using peripheral quantitative CT, and bone lengths via linear measurements. Infant mountain gorillas reach adult-like femoral/humeral length proportions before the onset of independent locomotion. Strength proportions, however, vary with locomotor behavior: arboreal infants (under 2 years of age) have relatively stronger humeri compared to femora than juvenile or adult mountain gorillas (p < .001), but are not different from adult lowland gorillas (p = 0.97). Additionally, the ulna becomes relatively stronger than the radius with age in mountain gorillas, which may be related to ontogenetic changes in hand posture during the transition to knuckle walking.

These results agree with the suggestion that length proportions reflect longer-term (genetic) adaptation to particular behaviors, while strengths reflect the immediate mechanical environment. Studying ontogeny of skeletal properties and locomotion conjointly allows for a better understanding of the determination of adult skeletal differences, as well as the relationship between morphology and behavior.